The present invention relates to the manufacture of mirrors, including inexpensive, lightweight mirrors having molded plastic substrates, and more particularly to increasing the thermal stability and resistance to distortion of such mirrors in the pressure at thermal variations and exposure to thermal radiation. Mirrors utilizing this invention can be used at any wavelength in particular for infrared application between 7 and 14 microns.
Precision mirrors require a specified mirror shape, such as flat, spherical or other contoured shape, to be maintained to a high degree of accuracy. The degree of precision required varies depending on the application. For the present mirrors, accuracies of 0.1 to 20 wavelengths of HeNe Light (.lambda.=0.6328 microns) are typical. In order to beneficially use the energy incident on a mirror it must be reflected to the greatest degree practical. A mirror coating is used for this purpose. Requirements for reflectance typically vary from 85 to 99% depending upon the application and wavelength region. These coatings, by definition, have a low emissivity since emissivity can be no greater than 1-reflectivity. Since absorption and emissivity are related, a low reflectivity (no coating) surface would have a high emissivity and therefore be highly absorbing of incident energy. The reduction of this absorption provides the basis for the present invention.
Absorbed radiative energy on the back (non mirror) side can generate thermal gradients caused by non-uniform heating resulting from the mismatch in energy being absorbed by the high emissivity (uncoated) back surface of the mirror substrate, of glass or molded plastic, and the low emissivity reflective coating on the front surface of the mirror substrate.
Non-uniform heating can result in a thermal gradient which can warp the mirror substrate or cause a loss of dimensional stability, and thereby distort the mirror figure and reflected light. This effect can occur with any mirror substrate, including glass, but is particularly pronounced with molded plastic mirror substrates because they are particularly sensitive to thermal problems associated with a low thermal conductivity and a high temperature coefficient of expansion (CTE) as is inherent in most plastics. Therefore while molded plastic mirror substrates provide low cost, lightweight alternatives to glass mirror substrates, they are much more susceptible to warpage and distortion caused by thermal gradients and therefore have been unsatisfactory for use in precision mirrors, particularly those used in heat build-up or radiant energy conditions.